The present invention is related generally to a servo-valve control device and a servo-valve control system, and more specifically, to such a device and a system which can operate stably even when a disturbance has emerged at the input terminal of the servo valve.
Servo valves have been used for various purposes including turbine speed control devices in thermal power plants. As for a turbine speed control device, for example, as shown in FIG. 13, a turbine revolution speed NR and a command of turbine revolution speed Ns are input to a revolution speed controller 1. Generator load GL and load command Gs are input to the load controller 2, and command signal of flow rate of a main-steam control valve is calculated. The flow rate command signal is input to a flow-rate-to-valve-opening conversion function 3 for converting the flow rate command signal to a valve opening command signal 3e. Here, the expression of “valve opening” is interchangeably used as “valve position”.
The valve opening command signal 3e and an actual valve opening signal 4e which has been detected by a valve-opening detector 4 are input to a servo-valve control device 5. The two signals 3e and 4e are compared in the servo-valve control device 5 and the difference is output to a solenoid (or servo coil) 7-1 of a servo valve 7 as a servo command signal 5e via a valve interface 6.
The servo coil 7-1 converts the servo command signal 5e into an oil pressure with an electric-to-oil-pressure converter (not shown). The converted oil pressure is transmitted into an oil cylinder, and a piston in the oil cylinder is moved to change the main-steam control valve opening.
FIG. 14 shows internal functions of a prior-art servo-valve control device 5. The servo-valve opening command signal 3e and the actual valve opening signal 4e are input to a summer 5-1 for outputting the difference. The difference is multiplied by a valve-position control gain in a power amplifier 5-2. Then, an output of a null bias compensator 5-3 is added to the output of the power amplifier 5-2 in a summer 5-4. The output of the summer 5-4 is, then, input to a limiter 5-5. The output of the limiter 5-5 is input to a valve interface 6 as a servo command signal 5e. The output of the valve interface 6 is input to a servo coil 7-1 so that the servo valve 7 is driven.
The null bias compensator 5-3 provides a bias for controlling the main-steam control valve to the fail-safe side or to the valve closing direction when the servo current to the servo coil 7-1 is lost. The limiter 5-5 is optionally disposed for limiting the servo command signal that is output of the controller.
FIG. 15 is a block diagram of a 3-coil servo system, which has a triplex structure of systems of A, B and C for a single main-steam control valve 8, in order to enhance reliability of the servo-valve control device 5. The triplex structure of systems of A, B and C includes servo-valve control devices, valve interfaces, servo coils and valve-opening detectors.
As shown in FIG. 15, outputs 4Ae, 4Be and 4Ce of valve-opening detectors 4A, 4B and 4C in the systems A, B and C, respectively, are all input to middle value gates 5-M in servo-valve control devices 5A, 5B and 5C. Each of the middle value gates 5-M outputs the middle value in the inputs. Then, the middle value is compared with the servo-valve opening command signal 3e, and the difference is output in the summer 5-1, as have been discussed referring to FIG. 14. The rest parts of the functions are same as that disclosed in FIG. 14, and not repeated here.
The outputs of the servo-valve control devices 5A, 5B and 5C are input to the middle value gates 6-1 of the valve interfaces 6A, 6B and 6C. The middle value gates 6-1 are of the same construction of the middle value gates 5-M described above, and the middle values are selected and output there. The outputs of the middle value gates 6-1 are amplified by the amplifiers 6-2 in the valve interfaces 6A, 6B and 6C.
In the 3-coil servo system shown here, the servo currents are directly detected by the servo-current detectors 7-2A, 7-2B and 7-2C disposed at the servo valves 7A, 7B and 7C, respectively, and are fed back. Thus, the abnormal condition in the valve interfaces 6A, 6B and 6C is detected from the servo current signals, and a circuit separation command is output to one of the circuit separation switches 6-3A, 6-3B and 6-3C. Thus, the abnormal valve interface is separated, as disclosed in Japanese Patent Application Publication. (Tokkai) Hei 4-228839.
In the servo-valve control device of the prior art such as that disclosed above, proportional control is used. In such a device, control deviation may be generated between the main-steam control valve opening command and the actual valve opening, due to mechanical null bias movement of the servo valves and various input terminal disturbances in the servo valve mechanisms. Such a control deviation may deteriorate the control performance of the servo-valve control device.
Therefore, null bias compensation value must be tuned periodically. Furthermore, if an input terminal disturbance to the servo valve has occurred in a 3-coil servo system, control deviation may be generated between the main-steam control valve opening command and the actual valve opening. The input terminal disturbance may include a one-system abnormality in the valve interfaces, and one-system or two-system disconnection of the servo coils. Such a control deviation may cause deterioration of the control performance of the servo-valve control device.
A typical approach to elimination of such control deviation is addition of an integration control. However, the servo valve, which is to be controlled, has a characteristics of integration in the relation between the control input (or the servo current) and the observation output (or the main-steam control valve position). Therefore, if an integrator is added to the controller side, the closed loop response would become slower, and the stability might deteriorate.
In the 3-coil servo system of the prior art described above, the servo current of each system is directly detected by its respective servo-current detector. Thus, the system is identified where the power-amplifier abnormality, servo-coil disconnection etc. has occurred. Then, the output from the abnormal system is separated, and normal control is continued. However, the servo-current detectors 7-2A, 7-2B and 7-2C and the circuit separation switches 6-3A, 6-3B or 6-3C are required for each system to be constructed, which results in high cost hardware. In addition, reliability of the total system is lowered, considering the failure of the servo-current detectors 7-2A, 7-2B and 7-2C and the circuit separation switches 6-3A, 6-3B and 6-3C.